1. Field of the Invention
The present invention relates to the field of data communication, and particularly to a jitter detection method and apparatus.
2. Description of the Related Art
Communication networks are commonly used to interconnect a variety of computing devices. These networks typically include a number of communication nodes that transmit data from their encoders to their decoders across a variety of transmission media. For example, a number of communication protocols (such as the Ethernet protocol and the Token Ring protocol) utilize twisted pair cables (such as category three unshielded twisted pair cables (UTDCAT3)) to transmit data from their encoder to their decoder.
Transmission of data across transmission media causes the phase and amplitude of the data to be distorted. Amplitude and phase distortions in turn can cause decoders to erroneously predict the value of the received data. For example, phase distortions can cause correlated jitters (i.e., can cause phase-related abrupt spurious variations in the phase of successive received data symbol pulses with respect to the phase of a continuous reference clock), which reduce the probability that demodulators of the decoders will accurately predict the value of the received data by preventing the demodulators from sampling the received signals at the center of the data symbol pulses.
Numerous prior art equalizers have been developed to compensate for phase and amplitude degradations of the transmitted data. For example, prior art equalizers include analog discrete equalizers and digital equalizers (such as automatic line build-out, finite impulse response (FIR), and infinite impulse response (IIR) equalizers). In addition, a number of these prior art equalizers adjust their equalization adaptively. In other words, a number of these equalizers increase their phase and amplitude compensation as the phase and amplitude distortions of the transmitted data increases. One example of an equalizer that adaptively adjusts its phase and amplitude compensation is disclosed in the U.S. Patent Application Ser. No. 08/414,618, filed Mar. 31, 1995, entitled "A Quasi-Adaptive Analog Equalization Method And Apparatus," now U.S. Pat. No. 5,717,716.
Furthermore, in order to ascertain when additional phase and amplitude compensation is needed to oppose the increasing phase and amplitude distortions, some adaptive equalization schemes (such as the equalization scheme disclosed by the above-noted patent application) utilize jitter detection circuits for determining when phase distortions exceed predetermined maximum values. Consequently, there is a need for an innovative jitter detection method and apparatus for detecting when the correlated jitter of the transmitted data exceeds a maximum tolerable value in order to inform an adaptive equalizer to adjust its phase and amplitude compensation.